There are significant pressures for =
EU-wide=20
modelling=97 but what are the current capabilities?=20

Europe is poised to establish regulations to limit the =
concentration of=20
particulate matter (PM) in ambient air under the auspices of the =
Air=20
Quality and Assessment Directive. PM10, =
PM2.5 and PM0.1 are defined =
as=20
particulate matter with a diameter smaller than 10 =B5m, 2.5 =B5m =
and 0.1 =B5m=20
respectively. These regulations are intended to set mandatory air =
quality=20
standards with which Member States will be expected to comply, yet =
a=20
number of recent studies suggest that elevated particulate =
concentrations=20
in urban areas during pollution episodes are related to non-local =
or even=20
transboundary pollution. These studies raise serious concerns =
about how=20
best to control ambient concentrations of PM, and about the =
effectiveness=20
of national air quality regulations to control exposure to =
particulate=20
matter in urban areas. At the same time, certain Member States =
have=20
expressed a wish to carry out modelling exercises to explore the=20
cost-effectiveness of vehicle emission abatement measures as part =
of the=20
Auto/Oil-2 Programme.

This article provides an overview of new initiatives in the =
area of=20
particulate matter modelling that are aimed at unravelling these =
difficult=20
issues.

PM MODELLING=97THE ESSENTIAL BUILDING =
BLOCKS=20

As with the modelling of other pollutants, robust modelling of =
future=20
particulate concentrations initially requires the following input =
data:=20

base year emission inventories and spatial distribution;=20

base year validation data; and=20

projections for future emission trends.

The particulate issue is greatly complicated by the need to =
consider=20
not only man-made and natural sources of particulates that are =
emitted=20
directly to the atmosphere (primary particulates), but also =
particulates=20
formed in the atmosphere through physical and chemical reactions=20
(secondary particulates). Secondary particulate matter includes =
inorganic=20
aerosols (ammonium sulphate and ammonium nitrate), and organic =
aerosols,=20
in which case changes in non-PM precursor emissions are important. =
Resuspension of dust that had settled onto surfaces and =
transboundary=20
transport of PM add further layers of complication.

EMISSION INVENTORIES

Emission inventories are only available from a small number of=20
countries including Germany, The Netherlands and the UK. Each has=20
different source categories and none are comprehensive in their =
coverage=20
of all type of sources. They may also be limited in their spatial=20
coverage, e.g. confined to single cities. The estimated relative=20
proportions coming from each source type also vary between =
different=20
inventories. As yet, the relationship between emission estimates =
and=20
ambient air quality is largely unconfirmed by source attribution =
analysis.=20
Table 1 provides a comparison of the status of various prominent=20
inventories.

Comparison of prominent particulate =
matter=20
inventories=20

emission inventory

spatial coverageand=20
disggregation

primaryPM

secondaryPM

resuspensionof PM

TNO

approximately 50x50 km Europe =
grid

man-madePM10, PM2.5 & PM0.1

none

none

UK

national

man-made PM10

none

none

Birmingham

1x1 km West Midlands grid

man-made PM10

none

none

London

1x1 km Greater London grid

man-made PM10

none

none

Germany

national

man-made PM10

none

none

Netherlands

national

man-made PM10

none

=

none

Table 1 Emission inventories are =
only=20
available from a small number of countries; so far their =
reliability=20
remains questionable.

Limited PM emission data means that, for Europe as a whole, PM =
source=20
apportionment relies heavily on judgement and it is widely =
recognized that=20
current estimates are subject to significant error. Where emission =
inventories are derived from energy consumption figures, =
assumptions also=20
need to be made concerning the extent to which abatement measures =
are=20
applied. The basis for forward projections of emissions is also a =
source=20
of uncertainty.

MODELLING OF AMBIENT PM=20
CONCENTRATIONS

A number of models are beginning to be examined as possible =
candidates=20
for modelling particulate matter across the European Union. These =
range=20
from complex dispersion models to simple empirical relationships =
derived=20
from the comparison of emissions and air quality. The detailed=20
capabilities of the models are important. In all cases they rely =
on=20
questionable emission inventories. Some only model certain types =
of=20
particulate, whilst others are not validated or validated for only =
a=20
limited number of cities. Although much work is currently under =
way to=20
improve this situation, so far, no single model can be considered =
reliable=20
and comprehensive in its treatment of PM at the EU scale. Thus, =
the=20
results from any such modelling work need to be treated with =
extreme=20
caution.

VALIDATION OF MODELLING RESULTS=20

Direct measurements of PM10 are =
necessary to=20
validate the models. PM10 data are scarce =
in all=20
countries and, to the best of our knowledge, data have only been =
collated=20
at the EU level from France, Germany, Luxembourg, Netherlands, =
Spain,=20
Sweden and the UK, totalling 70 monitoring sites across the EU as =
a whole=20
(all types of sites). PM2.5 data are even =
more=20
scarce, with data collated at EU level limited to only a few sites =
in a=20
few countries. In addition, there are no standard methods for =
measuring=20
PM10 or PM2.5 and, =
as a=20
consequence, measurements using different methods are not directly =
comparable. For instance those measurements which employ heated =
elements=20
may lose the volatile particulate fractions, and certain =
measurement=20
methods capture a narrower range of particles than other methods, =
e.g.=20
more of the smallest size fractions escape through the sampling=20
device.

CONCLUSIONS

No single model is comprehensive in its treatment of =
particulate matter=20
and there remains a real need to structure models that can address =
all=20
man-made and natural sources of primary and secondary particulate=20
matter.

Moreover, the uncertainties associated with current modelling=20
capabilities for particulate matter are considerable, and these=20
uncertainties will be added to the uncertainties surrounding the =
health=20
impacts of particulates. Validation of the emission inventories =
and models=20
will be crucial in the early stages of attempting to explore this =
complex=20
area using air pollution models.

Under these circumstances, the legislator should set air =
quality limit=20
values for PM at a pragmatic level. In particular, some allowance =
should=20
be made in the initial stage for occasionally exceeding limit =
values. Once=20
further research has reduced the current uncertainties, a =
tightening of=20
this allowance might be justified.